no on the 9600 GT and GSO.
I think you will be ahead of the game to get a single, more powerful card and then in a few months, you will most likely be able to pick up a second one if you feel the need at a much cheaper price. the 260 is a great card. cant think of a knock against it other than the price shot back up on it when it could be had much cheaper just a few months ago.
PhysX is one of Nvidia's main selling features. Basically it uses a GPU to compute phsyics calculations, taking some of the load of your CPU. Normally if you're SLI-ing 2 cards they need to be exactly the same, but with PhysX its ok to have one card to the GPU rendering (that would be ur GTX260) and a less capable (but PhysX enabled) other card can do the PhysX calulations.
Another perk (if I'm not mistaken) is that you can use the 2'nd card's video outputs and add extra monitors if you want (on top of the usual 2 monitors). With Nvidia's Fermi launch (new line of cards) not far off, I'd try to sell my cards quick b/c prices on older models might go down.
PhysX doesn't take any load off the CPU. Physics is usually CPU-side, but nVidia's PhysX is completely GPU. It's not worth it if you don't play many of nVidia's games.
Cypress, if you have Windows7 and are still looking for a PhysX setup, then do an HD5830/9600GSO combination. Priced at $240, should be right in your budget (especially if you sell the 8800GT/9600GT) [This is keeping in mind the GTX260 is $210] ).
Sorry my bad, let me clarify. PhysX performs calculations similar to those done by the CPU. It will not "take-over" calculations from the CPU, rather it will add its own power to it in games that are PhysX enabled. "Heavy computational tasks are done on the GPU (spell particles, leaves), more regular rigid body simulations (ragdolls) are calculated on the CPU simultaneously. So in a sense, the PhysX GPU's computations add to the effects in certain games... http://physxinfo.com/data/faq.html#1_1